For many years the general public has been concerned about the possible health effects of exposure to electromagnetic radio frequency (RF) radiation. Although research is ongoing and there appears to be no scientific consensus on the subject at this time, there have been concerns expressed that high levels of radiation fields may cause a variety of physical effects on the human body. With the dramatic increase in public use of wireless communication devices, and particularly mobile telephones, it has become prudent to consider that these products do not expose their users to levels of radiation that may be excessive. At the frequencies at which most of these devices operate, the known physiological effects center around tissue heating. A measure of this heating effect is known as Specific Absorption Rate (SAR), which is defined as the time derivative of the incremental energy (dW) absorbed by or dissipated in an incremental mass (dm) contained in a volume (dV) of a given density (ρ).
As part of worldwide efforts to legislate on consumer health and safety aspects, many regulatory bodies in the United States of America as well as abroad (e.g., governmental agencies such as the Federal Communications Commission or FCC in the United States) now require products that are placed on the market to meet SAR limits. Measurement of SAR has therefore become mandatory for companies that make such products.
The basis for US requirements may be found in ANSI/IEEE C95.1 “Safety Levels with Respect to Human Exposure to Radio Frequency Electromagnetic Fields, 3 kHz to 300 GHz” which establishes exposure limits, and ANSI/IEEE C95.3 “Recommended Practice for the Measurement of Potentially Hazardous Electromagnetic Fields—RF and Microwave”. These standards are reflected in the current FCC requirements found in 47 CFR §2.1091 and 2.1093. OET Supplement C 01:01 “Evaluating Compliance with FCC Guidelines for Human Exposure to Radiofrequency Electromagnetic Fields” gives guidance on the application of the FCC rules. (OET is the FCC's Office of Engineering and Technology). The FCC rules for evaluating portable devices for RF exposure compliance are contained in 47 CFR §2.1093. For these purposes, a portable device is defined as a transmitting device designed to be used with any part of its radiating structure in direct contact with the user's body or within 20 centimeters of the body of a user or bystanders under normal operating conditions. For distances greater than 20 centimeters, exposure evaluation is determined by the maximum permissible exposure limits (MPE) provided in OET 65.
For instance, the FCC limit for exposure from cellular telephones is a SAR level of 1.6 watts per kilogram (1.6 W/kg) averaged over 1 gram of tissue. The SAR scan test is usually 6 minutes for most technologies except for those operating in the range of 5 GHz to 6 GHz. Tests for technologies operating at such higher RF levels typically require a smaller step size for the necessary volume scan, thereby giving rise to a testing time of about 15 minutes.
Because of the ever-improving advances in the wireless communication technologies, the regulations for limiting electromagnetic RF radiation are being continuously updated as well. With the change in usage of mobile phones and the concomitant advent of newer technologies, further regulatory changes are anticipated. Currently, with the goal of reducing the likelihood that cellular phone transmissions of electromagnetic radiation will cause harmful effects in users, government regulations in several countries limit the maximum power level with which the cellular phones can radiate. This limitation is tied to a SAR threshold. However, the SAR level for a wireless device is determined in a common, specified manner for all cellular phones, and if a proposed phone design exceeds the SAR threshold, the design of at least part of the RF transmitter system must be adjusted. These adjustments, if not handled well, can be detrimental to the efficiency and performance of the phone's RF systems.